Mystery of the 5-Legged Freaks

A Purdue study found deformities in 8 percent of the 2,000 tiger salamanders examined, like this five-legged individual photographed in April 2004. Purdue researcher Rod Williams has ruled out inbreeding, which has been linked with elevated rates of deformity in a wide variety of animals, as a cause.Credit: Purdue University Forestry and Natural Resources

Oddly high rates of deformation among salamanders, frogs and other
amphibians have puzzled scientists in recent years. Possible causes
include inbreeding, parasites or pollution that permeates the animals' skin.

The mysterious deformations have included five-legged salamanders, frogs with extra limbs and other amphibians born so messed up that they can't survive to reproduce. In at least some cases, frog deformities are related to increasing numbers of infections
caused by tiny parasites that thrive on nutrient-rich runoff from North
American farms, a study last year
found.

Now, a study of tiger salamanders concludes inbreeding is not the
cause in their case, further pointing to human activity as the culprit
in amphibian deformities across the board.

Researchers examined 2,000 adult and juvenile salamanders and found
8 percent had deformities, mainly consisting of missing, extra or
dwarfed digits (fingers and toes). That's similar to the rate seen in
many frog species, said Purdue University Assistant Professor of Forestry and Natural Resources Rod Williams.

Like many types of amphibians, tiger salamanders return to the same pond throughout their lives to mate. Williams and his former doctoral adviser, lead author Andrew
DeWoody, hypothesized that habitat fragmentation or other factors might
increase the probability that related salamanders would return to the
same spot and mate, meaning more inbreeding and then maybe more deformities.

But their study found animals' genetic backgrounds to be unrelated
to deformation rates; deformed salamanders were no more inbred than
normal individuals. The population proved to be quite diverse, in fact,
with roughly twice as much genetic variation as most land animals,
DeWoody said.

"This is really the first study to test — and disprove — the
hypothesis that inbreeding is responsible for malformations in
salamanders," Williams said. The results were published in the journal Biology Letters.

High rates of amphibian malformation concern scientists because they threaten the survival of certain important species. And as with the alarming and widespread decline of frogs tied to global warming, the plight of amphibians is thought to suggest something about the overall health of the environment.

"Amphibians are a good bio-indicator species — real canaries in the coal mine," Williams said.

The mystery is not yet solved.

"We've crossed out inbreeding as a possibility, an important step
forward," DeWoody said, "but there's a lot of work yet to do."

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Author Bio

Robert Roy Britt

Rob was a writer and editor at Space.com starting in 1999. He served as managing editor of Live Science at its launch in 2004. He is now Chief Content Officer overseeing media properties for the sites’ parent company, Purch. Prior to joining the company, Rob was an editor at The Star-Ledger in New Jersey, and in 1998 he was founder and editor of the science news website ExploreZone. He has a journalism degree from Humboldt State University in California.